While Bitcoin mania has overtaken casual gambling addicts and Ponzi schemes of late, established figures in technology and finance have been much more cautious. Although Gates doesn’t seem to think that Bitcoin is as much of a fraud as other investors, he thinks it poses significant problems for governments worldwide.

“The main feature of crypto currencies is their anonymity,” Gates wrote in a response to what he thinks of crypto. “I don’t think this is a good thing. The Governments ability to find money laundering and tax evasion and terrorist funding is a good thing. Right now crypto currencies are used for buying fentanyl and other drugs so it is a rare technology that has caused deaths in a fairly direct way. I think the speculative wave around ICOs and crypto currencies is super risky for those who go long.”

Cryptocurrencies — particularly Bitcoin — aren’t innately anonymous, as every transaction is documented in a publicly-viewable leger, making the currency more transparent (in certain ways) than cash. But crypto also lacks any of the government regulation that’s required for large cash transactions, making it feasible for criminals to cash out cryptocurrencies in secret. That’s likely what Gates is referring to.